Raw detector signals of individual pixels, for example of a pixel matrix, of a radiation detector usually exhibit pixel noise. Under normal conditions, the pixel noise is determined by the finite number of the radiation quanta striking the radiation detector, and thus by the quantum statistics, and by electronic noise of electronic components. The electronic noise forms a significant component of the pixel noise in the event of high attenuation of the radiation by an object to be examined, that is to say in the event of low average quantum energies of the quanta striking the radiation detector.
By contrast with the quantum noise, which has a Poisson distribution and therefore always leads to raw detector signals with positive signal values, electronic noise can lead to signal values less than zero. Negative signal values lead to image quality problems that are known, for example, by the term “clipping”.
Methods for noise reduction and for avoiding clippings are known in which the negative signal values are replaced by physically sensible values, for example by one signal value or by a combination of a number of signal values of adjacent pixels. Attenuation values, also termed projections, are respectively calculated from the signal values postprocessed in this way. In simplified terms, in order to determine the attenuation value the logarithm of the ratio of “unattenuated signal value” to an attenuated, that is to say measured, signal value of the pixel is respectively calculated. The attenuation values are then subjected to noise reducing filtering for the purpose of noise reduction. The filtered attenuation values form the basis for determining an attenuation image which, in the case of x-ray computed tomography, for example, is obtained by a filtered back projection.
A disadvantage in this case is that the attenuation images exhibit corrupted quantitative information. By way of example, this in turn impairs the diagnosis in the case of radiological examinations.